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Dynamic Oxidative Potential of Atmospheric Organic Aerosol under Ambient Sunlight Huanhuan Jiang, and Myoseon Jang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00148 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Dynamic Oxidative Potential of Atmospheric Organic Aerosol under Ambient Sunlight

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Huanhuan Jiang† and Myoseon Jang†*

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Infrastructure and Environment, University of Florida, Gainesville, FL 32608, USA

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Correspondence to: Myoseon Jang ([email protected])

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ABSTRACT

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The atmospheric process dynamically changes the chemical compositions of organic aerosol (OA),

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thereby complicating the interpretation of its health effects. In this study, the dynamic evolution of the

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oxidative potential of various OA was studied, including wood combustion particles and secondary

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organic aerosols (SOA) generated from different hydrocarbons (i.e. gasoline, toluene, isoprene and α-

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pinene). The oxidative potential of OA at different aging stages was subsequently measured by the

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dithiothreitol consumption (DTTm, mass normalized). We hypothesized that DTT consumptions by

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OA were modulated by catalytic particulate oxidizers (e.g., quinones), non-catalytic particulate

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oxidizers (e.g., organic hydroperoxides and peroxyacyl nitrates) and electron-deficient alkenes. The

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results of this study showed that the oxidative potential of OA decreased after an extended period of

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aging due to the decomposition of particulate oxidizers and electron-deficient alkenes. Quinones (GC-

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MS data) partially attributed to the DTTm of fresh wood smoke particles but rapidly dropped with

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aging. In biogenic SOA, organic hydroperoxides (4-nitrophenyl boronic acid assay) exclusively

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accounted for DTTm and decreased with aging. The DTTm of aromatic SOA, mainly comprising

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organic hydroperoxides and electron-deficient alkenes (FTIR data), was shortly elevated during the

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early atmospheric process, however, showed a noticeable decrease (32-75%) for a long period of

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aging. We concluded that fresh or moderately aged OA are more reactive to a sulfhydryl group than

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highly aged OA.

Department of Environmental Engineering Sciences, Engineering School of Sustainable

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TOC figure

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1. INTRODUCTION

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Exposure to particulate matter (PM2.5, aerodynamic diameter < 2.5 µm) has been implicated in the

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detrimental cardiovascular and pulmonary diseases (e.g. asthma, and myocardial infarction).1-3

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Organic aerosol (OA) comprises a substantial fraction (20-90%) of PM2.5,4 however, the health effect

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of OA remains unknown. OA is a complex mixture of organic compounds and contains a large fraction

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of particulate oxidizers, which may react with cellular antioxidants (e.g. glutathione peroxidase-1;

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GSH), thereby interrupting the oxidative balance and leading to a cascade of oxidative stress

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responses.5

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A chemical assay using dithiothreitol (DTT), a surrogate of biological reducing agents, has been

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widely applied to measure the capability of PM2.5 to oxidize cellular materials. Catalytic particulate

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oxidizers like quinones can efficiently oxidize DTT through a redox cycle and have been recognized

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as the major contributors to the oxidative potential of PM2.5.6, 7 However, the fraction of quinones in

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OA is low, especially in secondary organic aerosols (SOA).8-11 Our previous studies have reported the

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importance of non-catalytic oxidizers (e.g. organic hydroperoxides (OHP) and peroxyacyl nitrates

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(PAN)) and electron-deficient alkenes in DTT responses of OA.12, 13 Congruently, OHP (including

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alkyl hydroperoxides and acyl hydroperoxides) and PAN can oxidize DTT to disulfides, sulfenic acids,

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sulfinic acids or sulfonic acids through non-catalytic reactions.14, 15 An electron deficient alkene is

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defined as the C=C (alkene) double bond coupled with an electron withdrawing group, such as a

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carbonyl, a nitro, and a carboxylic acid.16 An electron-deficient alkene can react with a sulfhydryl

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group in DTT via a Michael addition.16

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The fraction of particulate oxidizers in OA can be dynamically changed by the atmospheric process.

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During the initial aging process of hydrocarbons (HC) or OA, the molecular weight and the oxidation

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state of aerosol products presents an increase owing to the addition of oxygenated functional groups.

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After a long aging period, high-volatility compounds begin to form through the fragmentation of low-

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volatility compounds.17 The impact of the aging process on particulate oxidizers of OA has been

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studied using state-of-art techniques such as chemical ionization mass spectrometers and aerosol mass

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spectrometers.18 These studies, however, were limited to the tentative analysis of stable products due

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to the lack of standards and the instability of particulate oxidizers under high operation temperature of

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these instruments.

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The current work studied the aging effect on the oxidative potential of OA and its chemical

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compositions. OA is either derived from wood combustion smoke or produced from the

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photooxidation of HCs (i.e. gasoline, toluene, isoprene and α-pinene) in a large outdoor smog

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chamber. Gasoline, a highly volatile mixture of C4-C9 HCs, can easily evaporate into the atmosphere,

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and makes a large contribution (~4 Tg/yr) to global SOA.19 Wood combustion particles, containing a

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large amount of organic mass (e.g. oxygenated HCs and substituted aromatic HCs.), constitute 14% -

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30% of primary fine particles in the urban air.20-22 The oxidative potential of OA at different aging

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stages was measured using DTT assay. In order to study the significance of quinones in the oxidative

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potential of OA and their stability with photochemical aging, quinones in wood smoke particles and

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gasoline SOA were measured by gas-chromatography mass-spectrometry (GC-MS) over the course

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of the chamber experiment. OHP and PAN compounds in OA were quantified using 4-nitrophenol

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boronic acid (NPBA) assay and Griess assay, respectively. The abundance of electron-deficient

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alkenes was analysed using a Fourier Transform Infrared (FTIR) spectrometer.

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2 MATERIALS AND METHODS

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2.1 Outdoor chamber experiments

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The photooxidation of HCs and wood smoke experiments were conducted at the University of Florida

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Atmospheric Photochemical Outdoor Reactor (UF-APHOR) with dual chambers (52 m3 each). The

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chambers were flushed using the clean air generated from the air purifiers (GC Series, IQAir) for two

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days prior to each experiment. Wood burning smoke particles were generated under either open-fire

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combustion (250 °C) or smoldering combustion (150 °C) using commercial hickory hardwood. For

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the experiments to form SOA from HCs (i.e. gasoline, toluene, isoprene, and α-pinene), liquid HC was

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injected into the chamber using a glass manifold with clean air. CCl4 was also introduced to monitor

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the chamber dilution. The desired volume of NO (2% in N2, Airgas) was injected using a syringe

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through the injection port connected to the chamber. All chemical species were introduced to the

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chamber before sunrise. A gas chromatography-flame ionization detector (HP 5890) was applied to

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measure the concentrations of HCs and CCl4. Concentrations of NOx and O3 were monitored using a

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chemiluminescence NO-NOx analyzer (Model 200 E) and a photometric O3 analyzer (Model 400 E),

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respectively. The relative humidity (RH) and temperature were measured using a hygrometer (the

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Campbell Scientific, CS215-L). The sunlight intensity was monitored inside the chamber using the

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ultraviolet radiometer (TUVR, the Eppley Laboratory, wavelength 290-385 nm). Particle number

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concentration was monitored using a scanning mobility particle sizer (SMPS, TSI, Model 3080)

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coupled with a condensation nuclei counter (TSI, Model 3025A and Model 3022). The particle number

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concentration was converted to the mass concentration using the density of OA (1.3 g cm-3 for -

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pinene SOA and 1.4 g cm-3 for other OAs).11, 23, 24 The experimental conditions and SOA yields are

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listed in Table 1. The time profiles of concentrations of OA mass, NOx, NO, and O3 are shown in

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Figure S1.

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2.2 Sampling and extraction methods

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A particle-into-liquid sampler (PILS) coupled with a carbon denuder (to remove gaseous compounds)

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was utilized to collect particles within a small amount of water at an air flow rate of 13 L/min.25 The

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PILS collection efficiency is larger than 95% for particles in this study.25 The PILS samples were

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subsequently applied to the chemical assays described in Section 2.3.

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The Teflon filter (13 mm diameter, Pall Life SciencePallflex, type TX40HI20-WW) was applied to

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collect wood smoke particles or gasoline SOA at a flow rate of 28 L/min with the use of a pump.

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Immediately after sampling, a 10-L of internal standard, comprising of six deuterated PAHs, was

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added to the filter. The details of the internal standard can be found in Section S2.1, Supporting

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Information (SI). The particles were then incubated in 20 mL methylene chloride for 6 h at room

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temperature to extract the organic compounds. Metals and black carbon were excluded due to their

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poor solubility in methylene chloride. The extraction efficiency of particles was about 40-80%, which

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was estimated using the filter mass before and after solvent extraction. The filter-extract was

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subsequently concentrated to 0.3 mL using dry air, transferred to a GC vial, and applied to the GC-

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MS analysis.

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2.3 Chemical assays

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The QA/QC data of DTT assay, OHP analysis, and PAN analysis can be found in our previous study.13

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The DTT measurement of wood smoke particles utilized the filter-extract samples, and all other

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chemical analysis used the PILS samples.

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2.3.1 DTT assay

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The oxidative potential of OA was measured using the DTT assay. A reaction mixture of 1.9 mL filter-

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extract or PILS sample, 0.6 mL potassium phosphate buffer (25 mM), and 0.5 mL DTT (0.875 mM)

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was incubated at 37 °C and continuously shaken using an Edison Environmental Incubator Shaker

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(G24) at a low shaking speed. After a specific incubation time (t, ranging from 50 min to 280 min), a

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0.5-mL of the reaction mixture was withdrawn and transferred to another vial, in which 0.5 mL

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trichloroacetic acid (1% w/v; the quenching reagent) was previously added. Then, 0.5 mL DTNB (1

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mM) and 0.5 mL Tris-base buffer (0.4 M, pH=8.9) were added to the quenched mixture. The residual

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DTT in the quenched mixture reacted with DTNB to form a yellow-color product, 2-nitro-5-

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thiobenzoic acid, which presents a high molar extinction coefficient (14150 M−1 cm−1) at 412 nm

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wavelength. The absorbance of the final mixture at 412 nm was measured using a UV/Vis

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Spectrometer (Lambda, PerkinElmer). Positive controls (0.25 μM PQN) and blank controls (DI water)

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were run in duplicates for each set of DTT measurements. The blank-corrected DTT consumption by

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OA (∆DTTOA, nmol) was calculated using the absorbance of DTT-blank mixture without incubation

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(A0), the absorbance of DTT-blank mixture after incubation (Ablk), the absorbance of DTT-OA mixture

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(AOA) after the reaction, and the initial DTT concentration (DTT0, nmol). ∆DTTOA =

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Ablk -AOA A0

DTT0

(1)

The mass-normalized DTT consumption, DTTm, is defined as, DTTm =

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∆DTTOA mOA

(2)

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where mOA is the OA mass applied to the DTT measurement. The DTT consumption was restricted

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within 50% of the initial DTT concentration by constraining the OA mass added to the reaction

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mixture.

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2.3.2 OHP analysis

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1 mL PILS sample was incubated with 100 μL NPBA solution (10 mM in methanol), and 900 μL

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KOH (50 mM) at 85 °C for 7 h. NPBA reacted with OHP to form a yellow-color product, 4-

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nitrophenol, which presents a large molar extinction coefficient (18000 M−1 cm−1) at 406 nm. Positive

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controls (20 μM H2O2) and blank controls (DI water) were run in duplicates for each set of

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measurements.

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2.3.3 PAN analysis

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PAN type compounds in 300 μL PILS sample were firstly hydrolyzed by adding 300 μL KOH (50

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mM) to form nitrites. Then, 1 mL Griess reagent (a mixture of 20 mM sulfanilic acid and 5 mM n-(1-

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naphthyl)ethylenediamine dihydrochloride) was added to react with nitrites forming azo dyes that

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could be quantified based on their absorbance at 541 nm wavelength. Positive controls (10 μM NaNO2)

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and blank controls (DI water) were run in duplicates for each set of measurements.

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2.4 GC-MS analysis

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Quinone compounds were analyzed using GC-MS. A 10-L of recovery standard (Section S2.2) was

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added to each GC vial before GC-MS analysis. A 1-μL filter-extract sample was injected in on-column

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mode to a Varian CP-3800 gas chromatograph interfaced with a Varian Saturn 2200 mass

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spectrometer. The column oven temperature was held at 45 °C for 0.5 min; then ramped to 100 °C by

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a 15 °C/min gradient and held for 2.5 min; and finally ramped to 280 °C with a 12 °C/min and held

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for 8 min. The individual compounds in particle extracts were identified using an external standard

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(Section S2.3) and tentatively analyzed using the National Institute of Standards and Technology

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(NIST) library. Only PQN and anthraquinone (AQN) were detected in the wood smoke particles.

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2.5 FTIR analysis

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The functionalities in toluene SOA was analyzed using an FTIR (Nicolet Magma 560, Madison, WI,

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USA). The chamber particles were collected on a silicon FTIR disc (13 × 2 mm, Sigma–Aldrich, St

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Louis, MO, USA) by impaction. The mass of collected particles was determined by measuring the

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FTIR disc mass before and after the sampling.

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3 RESULT AND DISCUSSION

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3.1 The effect of the atmospheric process on the oxidative potential of OA

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Traditionally, catalytic particulate oxidizers such as quinones in PM have been of wide interest due to

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their high production of ROS through catalytic processes.5 Assuming the pseudo-first order reaction

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of the DTT consumption by quinones, DTTm should linearly increase with reaction time t and the

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mass-normalized consumption rate of DTT should be constant.12 Hence, mass-normalized DTT

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consumption rate has been used to represent the oxidative potential of PM. However, if DTT is

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consumed via a non-catalytic process, DTTm is nonlinear to t.

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Figure 1 (1a-1b) and Figure 2 (2a-2b) illustrate the oxidative potential (DTTm) of various OA at

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different aging stages. As shown in Figure 1a, for freshly generated wood combustion OA, the DTTm

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of both high-temp and smoldering wood samples increased linearly with t, thereby suggesting that the

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DTT consumption originated mainly from the catalytic processes of quinones. However, the

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relationship between DTTm vs. t became deviated from linearity as wood smoke particles aged, thus

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suggesting that the DTT consumption was dominated by non-catalytic processes. Additionally, DTTm

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dropped by 52% for high-temp wood particle samples after a 9-h aging, and 74% for smoldering

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samples after a 7-h aging (Figure 1b). 9

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As shown in Figure 2a, the DTTm of gasoline SOA was found to be nonlinear to t during the entire

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period of SOA formation. It has been evidenced that gasoline SOA is mainly formed from the

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photooxidation of mono-aromatic ring HCs, such as toluene, (o, m, p)-xylene, 1,3,5-trimethylbenzene

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and 1,2,4-trimethylbenzene.26 Hence, in order to understand the behaviour of DTTm of aromatic SOA,

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the DTTm of toluene SOA produced under two different NOx conditions was measured. HNOX-

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toluene SOA showed a slightly higher DTTm than LNOX-toluene SOA (Figure 2a). Similar to gasoline

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SOA, the DTTm values of all toluene SOA samples were nonlinear to t. This implicates that DTTm

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was governed by non-catalytic DTT-reactive compounds. Additionally, as illustrated in Figure 2b, The

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DTTm of both gasoline and toluene SOA were influenced by the aging process. A 32% reduction in

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gasoline DTTm occurred after a 6-h aging. DTTm of HNOX-toluene SOA presented an increase in

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early aging time (before noontime) and then a sharp decrease (up to 50%) as SOA further aged in the

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afternoon. For LNOX-toluene, a significant decrease (up to 75%) of DTTm appeared over the course

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of the chamber experiment.

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Figure 2 also shows the DTTm of isoprene SOA and α-pinene SOA. The DTTm of biogenic SOA was

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nonlinear to t (Figure 2a) as expected in that there are no quinone products in biogenic SOA. Within

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our sampling period, a slight decrease of DTTm was observed between 12:15 and 14:25 for HNOX-

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isoprene SOA and a noticeable drop of DTTm appeared between 10:40 to 13:00 for LNOX-α-pinene

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SOA (Figure 2b).

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3.2 Evolution of particulate oxidizers

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To decipher the DTT consumptions by various OA, the chemical characteristics of particulate

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oxidizers in OA were determined. It was observed that the atmospheric process lead to the dynamic

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evolution of particulate oxidizers. As expected, quinones were revealed to be only significant in fresh

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wood smoke OA, but not in other OA. A considerable amount of OHP appeared in all OA, and OHP

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in SOA showed a dramatic reduction with a long-period aging process. However, the formation of

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PAN was insignificant in all the OA studied.

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3.2.1 Particulate oxidizers in wood combustion OA

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The time profiles of quinones (PQN+AQN) in wood smoke particles were determined using GC-MS

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analysis (Figure 3). There was a large amount of quinones in fresh wood smoke OA, which can well

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explain the linear increase of DTTm with t (Figure 1a). Compared to quinones (65 pmol/μg) in fresh

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smoldering OA, a higher concentration of quinones (80 pmol/μg) was observed in fresh high-temp

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OA. This result is in parallel with the higher DTTm as observed in fresh high-temp OA (Figure 1a).

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However, as particles aged, the concentrations of quinones presented a dramatic drop to a negligible

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amount, congruent to the nonlinearity between DTTm and t (Figure 1a). The aging effect on wood

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smoke particles has also been reported by Zhong et al. who observed a bleaching of chromophores

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(e.g. quinoids and phenols) with the progression of photooxidation. Quinones such as AQN can be

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photooxidized and decomposed to substituted-phenols, substituted-benzaldehydes, and ring-opening

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products with aging (Figure S2).27, 28

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Non-catalytic particulate oxidizers in wood smoke were also determined. As shown in Figure 1b, there

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was about 2 nmol/μg of OHP detected in wood smoke OA, and the OHP showed no significant change

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for long periods of aging. OHP contributed to 12-24 % of DTTm for high-temp wood samples and 28-

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78 % for smoldering samples (Figure 1c). Nevertheless, there were still unexplainable DTTm

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outcomes. For example, there was a large gap between DTTm and OHP in aged wood smoke particles,

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implicating the existence of other non-catalytic DTT-active species. Wood combustion has been

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reported to produce a large fraction (36-42%) of aromatic compounds,29 which can generate electron-

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deficient alkenes through the photooxidation.8,

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partially attributed to these electron-deficient alkenes. The importance of electron-deficient alkenes in

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wood smoke particles needs to be investigated further.

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3.2.2 Particulate oxidizers in gasoline SOA and toluene SOA

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Quinones were not detected in gasoline SOA (GC-MS data not shown).26 The quantity of OHP in

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aromatic SOA showed an increase in the morning while a noticeable decrease in the afternoon. For

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example, OHP in gasoline SOA increased from 2.3 nmol/μg (9:00) to 3.2 nmol/μg (11:30) and dropped

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back to 2.2 nmol/μg (14:40). OHP in HNOX-toluene SOA increased from 2.3 nmol/μg (10:20) to 3.5

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nmol/μg (12:30) and dropped to 1.2 nmol/μg (17:00). The quantity of PAN in aromatic SOA, though

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insignificant, showed a similar trend to OHP with aging. The concentration of organic peroxides

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(ROOR and OHP) in SOA formation have also been measured in other studies using the iodometric-

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spectrophotometric method. For example, Sato et al. reported a 17 wt % of organic peroxides in

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LNOX-toluene SOA. However, the NPBA assay with a high accuracy and robustness has been for the

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first time used to measure the OHP solely in our studies.13

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The increases of both OHP and PAN in the early stage of the aging process should be ascribed to the

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functionalization in the photooxidation products formed from aromatic HCs, as shown in reactions

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SR1-SR7, SI.31 Conversely, the decay of OHP and PAN can be explained by photolysis (reactions

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SR8 and SR9),32,

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(Scheme S1 and S2),10, 35, 36 or in-particle chemistry with other aerosol products (Schemes S3).37 The

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rate constant and corresponding half-life with respect to the decomposition of selected OHP and PAN

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compounds are summarized in Table S1.13, 32-35, 38-41 PAN is thermally unstable with a half-life ranging

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from 49 min to 55 min at 298 K.34 The reaction of PAN with OH is unimportant given its extremely

33

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DTTm of wood combustion particles could be

thermal decomposition (reactions SR10-15),34 the reaction with OH radicals

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low rate constant (e.g. 310-14 cm3/molecule/s for CH3C(O)OONO2 + OH).33 However, the major sink

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of OHP is attributed to its reaction with OH radicals. For example, the attack of TLBIPEROOH (a

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secondary product from the photooxidation of toluene) by OH radicals leads to a half-life of 35 min

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at 298 K.35, 36 The OHP may also be attacked by a carbonyl group via a Baeyer–Villiger reaction

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(Scheme S3).37 This reaction has been evidenced to be more preferable for aldehydes than ketones.37

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Multifunctional products containing both an aldehyde and a hydroperoxide group are abundant in

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toluene SOA and may allow the rapid intra-molecular reaction of a hydroperoxide with an aldehyde,

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leading to a rapid reduction of DTTm with aging.

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However, DTTm of aromatic SOA with aging can only be partially attributed to OHP and PAN. For

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example, as shown in Figure 2c, OHP (mostly) and PAN together contributed only 23-38% to DTTm

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in LNOX-gasoline, 22-35% in HNOX-toluene, and 25-73% in LNOX-toluene. Hence, the source of a

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large fraction of DTT response remained uncertain. We hypothesized that electron-deficient alkenes

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that could react with DTT through a Michael addition could be major contributors to DTTm of aromatic

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SOA. Electron-deficient alkenes refer to the alkenes coupled with electron-withdrawing groups (e.g.

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carbonyl and nitrates) and are commonly found in ring-opening products from the photooxidation of

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mono-ring aromatic HCs (Scheme S4).42, 43 To confirm their significance in DTTm, electron-deficient

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alkenes in toluene SOA were quantified using FTIR analysis. Figure 4a shows the concentrations of

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functionalities (C-H, O-H, C=O, C(=O)O-H, CH=CH) in LNOX-toluene SOA (mid-collection time

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of 15:00) that were estimated by decoupling the FTIR spectrum (Figure S3 and Table S2)42, 44. The

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mass fraction of OHP in LNOX-toluene SOA at 15:00 was estimated based on interpolation of OHP

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values in Figure 2b. The signal of organonitrate (1645 cm-1, 1559 cm-1, and 1340 cm-1) 42 in the FTIR

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spectrum was found to be negligible. Thus, we assumed that most C=C groups were conjugated with

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electron-withdrawing groups such as C=O and C(=O)O-H. This assumption was also supported by the 13

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alkene products, which were simulated using the MCM mechanism for toluene photooxidation.35, 36

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The total mole concentration of electron-deficient alkenes in LNOX-toluene SOA was estimated to be

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up to 5.0 nmol/μg-SOA, which can completely account for the gap between DTTm and OHP+PAN

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(Figure 4b). Hence, we imply that electron-deficient alkenes are important contributors to DTTm of

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toluene SOA, and presumably to that of gasoline SOA. The decrease of the gap between DTTm and

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OHP+PAN with a long period of aging thus can be inferred to have been caused by the decomposition

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of electron-deficient alkenes during further photooxidation processes (Table S1 and Scheme S5).35, 36,

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45

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3.2.3 Particulate oxidizers in biogenic SOA

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The double bond structures in isoprene and α-pinene are rapidly oxidized by OH radicals or ozone in

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the first stage of photooxidation,10, 46-49 thus leading to a low production of electron-deficient alkenes.

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Instead, biogenic SOA contains high concentrations of OHP compared to wood smoke particles and

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aromatic SOA. As shown in Figure 2c, DTTm of both isoprene and α-pinene SOA were exclusively

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attributed to OHP (mostly) and PAN. The decrease of DTTm with the photochemical aging was also

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in conjunction with the decrease of OHP. In detail, the concentration of OHP in HNOX-isoprene SOA

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was about 5.2 nmol/g at 12:15 (1st sample) with a slight drop in 2 h after the 1st sample, and that of

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LNOX-α-pinene SOA was 7.3 nmol/g at 10:40 (1st sample) with a dramatic drop to 3.8 nmol/g in

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3 h after the 1st sample. The decomposition of OHP, as explained in Section 3.2.2, was largely

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dependent on the reaction pathway via the attack by OH radicals. Correspondingly, Surratt et al.

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quantified the organic peroxides in LNOX-isoprene using the iodometric-spectrophotometric method

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and reported a reduction of organic peroxides in aged SOA.50 The amount of PAN (Figure 2b) was

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extremely low in HNOX-isoprene SOA due to the high volatility of low-molecular-weight PAN-type

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compounds in isoprene SOA (e.g. 0.5 Torr for peroxy methacrylic nitric anhydride).51 However, PAN

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compounds in LNOX-α-pinene SOA have a relatively high concentration (0.60 nmol/g at 10:40, and

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0.44 nmol/g at 13:05), probably attributed to their low vapor pressure (ranging from 10-7 to 10-3

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torr).52

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4 ATMOSPHERIC IMPLICATIONS.

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Traditionally, the mass-normalized DTT consumption rate has been measured to represent the

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oxidative potential of catalytic particulate oxidizers (e.g. quinones), which lead to a linear increase of

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DTT consumption with reaction time. However, the oxidative potential can also be processed by non-

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catalytic DTT-reactive compounds in PM. Correspondingly, when the DTT consumption rate is not

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constant with reaction time, it can mislead the interpretation of oxidative ability of PM. Thus, in order

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to discern the source of oxidative potential, the mass-normalized DTT consumption (DTTm) of various

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OA was measured with an extended reaction time, as demonstrated through this study. Only in fresh

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wood smoke aerosol, DTTm presented a linear increase with reaction time due to quinones (Figure 1a).

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For other OA including aged wood smoke and all SOA (Figures 1a and 2a), DTTm was nonlinear to

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reaction time, thus evidently showing that DTTm was mostly or completely promoted by non-catalytic

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DTT-reactive compounds. Hence, we suggest that DTTm is more suitable to scale the oxidative

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potential of a variety of OA than the mass-normalized DTT consumption rate.

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To understand the mechanistic role of OA in bearing the oxidative potential, the concentrations of

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non-catalytic particulate oxidizers (i.e., OHP and PAN) and electron-deficient alkenes were quantified

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using acellular assays or FTIR spectral data. In biogenic SOA, nearly 100% of DTTm was attributed

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to OHP (Figure 2c). For toluene SOA, in addition to OHP, electron-deficient alkenes were also

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important to increase DTTm. For example, in the LNOX-toluene SOA of this study, electron-deficient 15

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alkenes and OHP accounted for up to 67% and 37% of DTTm, respectively. Correspondingly, electron-

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deficient alkenes may also significantly contribute to DTTm of wood smoke particles and gasoline

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SOA. Through this study, a dynamic evolution of DTTm was observed with the photochemical aging

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(Figures 1b and 2b). Highly aged particles were found to be less reactive to the sulfhydryl group of

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DTT than the fresh or moderately aged particles (Section 3.1), corresponding to the decrease of

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particulate oxidizers (i.e., quinones, OHP, and PAN) and electron-deficient alkenes (Figures 1b, 2b,

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3, S2, Reactions SR8-15, Schemes S1-3 and Table S1). In the recent laboratory study, Krapf et al.

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reported that a significant amount of peroxide-containing oxygenated products was formed from

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ozonolysis of terpenes but these products were thermodynamically unstable (a lifetime in an order of

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hours).53 Additionally, the further oxidation of electron-deficient alkene products (Scheme S5 and

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Table S2) could also partially attribute to the decrease of the DTTm of OA such as wood smoke

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particles and aromatic SOA, but more direct evidence is needed to prove the importance of electron-

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deficient alkenes in the DTTm of aromatic-related SOA. In general, oxidative potential induced by PM

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is indirectly linked to the toxicity of PM as it can cause disorders in cellular signaling cascades, the

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development of inflammation, allergic responses and other respiratory diseases.54, 55 Based on our

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results, the toxicity of fresh or moderately aged OA may be greater than that of highly aged OA. The

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formation and the decay of particulate oxidizers and electron deficient alkenes of this study could

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severely affect the interaction of OA with health because these chemical species are major contributors

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to SOA mass in ambient air.

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The atmospheric environment in rural areas is mostly influenced by biogenic SOA, which has a high

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yield of OHP. While, conversely in urban areas the exhausted air plumes from gasoline- and diesel-

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powered motor vehicles are significant sources of primary OA (POA) and SOA precursors.56, 57 It has

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been estimated that motor vehicles contributed to 2.9±1.6 Tg-SOA/yr in the U.S. with a total urban 16

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emission of 3.1 Tg-SOA/yr,56 apparently suggesting that aromatic SOA is important in urban areas.

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Especially, the study of SOA from gasoline-powered motor vehicles has emerged to be as a critical

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imperative in recent years because of two reasons. First, the emissions of SOA precursors and POA

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from diesel-powered motor vehicles are largely reduced due to the application of catalyzed diesel

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particulate filters.56 Second, the OA derived from a gasoline vehicle is dominated by SOA based on

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recent chamber studies.56 Typically, the ambient OA under the urban atmosphere consists of various

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carbonaceous species originating from different aging stages including freshly formed species,

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moderately aged species and highly oxidized species due to the hydrocarbons continuously emitting

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from anthropogenic sources. Unlike the ambient air, chamber studies allow to measure the time series

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of the oxidative potential of OA, although only limited types of aromatic or gasoline SOA can be

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studied at a given NOx condition. Our studies of gasoline SOA, the characterization of DTTm and SOA

341

compositions, augment the understanding of the toxicity of PM produced in urban areas.

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The implication of this study is not limited to the oxidative potential of organic compounds in aerosol

343

phase. Given the high abundance of DTT-active species in the gas phase, the oxidative potential of

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gas-phase organic compounds should also be emphasized. For example, our previous study reported

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that the concentration of PAN in the gas phase of the isoprene+NOx system was 200 times higher than

346

that in SOA.13 Correspondingly, the concentration of OHP and electron-deficient alkenes in gas phase

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would also be high. Using the mathematical model, Davies reported that gaseous compounds can be

348

efficiently absorbed in the upper respiratory system (nearly 100%) due to their high diffusion

349

coefficients.58 Excess amounts of gaseous compounds beyond the clearance capacity of mucus in the

350

upper respiratory system, present a risk of being transported into the bloodstream and reaching the

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heart. The toxicological evidence is mounting that non-catalytic oxidizers are able to bind to the

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nucleophiles or nucleophilic sites in biomolecules, resulting in the DNA damage and the modification 17

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of proteins and lipids.59 For example, tert-butyl hydroperoxide, an OHP, is known to damage cells

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through either lipid peroxidation or the depletion of glutathione.59, 60 Conjugated carbonyls has also

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been recognized to induce toxicity in cellular materials through the formation of adducts with cysteine

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sulfhydryl groups in functional proteins or guanine residues in DNA.61, 62 Hence, studies on the toxicity

357

of both gas-phase and particle-phase non-catalytic oxidizers and electron-deficient alkenes are needed

358

in future.

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ASSOCIATED CONTENT

360

Supporting Information

361

The profiles of NOx, O3, temperature, and relative humidity in the outdoor chamber experimental

362

section; the workup procedures for GC-MS analyses; the evolution mechanisms of AQN, particulate

363

oxidizers and electron-deficient alkenes, the summary of the kinetic rate constant of the decomposition

364

processes of non-catalytic oxidizers and electron-deficient alkenes, the FTIR spectrum of LNOX-

365

toluene SOA, the functional group composition of LNOX-toluene SOA.

366

AUTHOR INFORMATION

367

Corresponding author

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*Phone: +1-352-846-1744; fax: +1-352-392-3076; e-mail: [email protected].

369

ORCID

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Huanhuan Jiang: 0000-0002-5581-375X

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Myoseon Jang: 0000-0003-4211-7883

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Notes. The authors declare that they have no conflict of interest.

373

ACKNOWLEDGEMENTS.

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The authors acknowledge the award from the Ministry of Science and ICT, the Ministry of

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Environment, the Ministry of Health and Welfare (2017M3D8A1090654) and the award from the

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National Institute of Metrological Sciences (KMA2018-00512).

377

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378 Table 1. Outdoor chamber experiment conditions. 379 Date and Experiment Condition b [HC]0 c [NOx]0 e OAmax Y RH g Temp g Chemical analysis h a Chamber (HONO) ppmC ppb µg/m3 % % K d 04/12/17 E Wood smoke High-Temp N/A N/A 2100 N/A 13-55 287-315 DTT,OHP,PAN,GC-MS 04/12/17 W Wood smoke Smoldering N/A N/A 1187 N/A 19-65 290-326 DTT,OHP,PAN,GC-MS 07/02/17 E Gasoline LNOX 20.0 470 (166) 212 29.8 f 14-50 297-324 DTT,OHP,PAN,GC-MS 05/29/17 E Toluene HNOX 4.5 764 (163) 254 11.5 16-54 296-320 DTT,OHP,PAN 05/29/17 W Toluene LNOX 5.2 319 (75) 280 13.9 26-64 297-316 DTT,OHP,PAN 09/19/17 E Toluene LNOX 7.6 609 (235) 344 13.6 11-47 292-322 FTIR 08/16/17 E Isoprene HNOX 15.0 3300 388 4.6 15-50 297-327 DTT,OHP,PAN 08/16/17 W α-Pinene LNOX 3.4 141 386 20.1 25-60 297-319 DTT,OHP,PAN a 380 “E” represents the east chamber and “W” represents the west chamber. b 381 Wood smoke was produced under either high-temperature (High-Temp) or smoldering combustion condition. SOA was 382 formed from the photooxidation of precursors under either high-NOx (HNOX) or low-NOx (LNOX) condition. c 383 [HC]0 represents the initial mixing ratios of HCs in ppmC. d 384 N/A represents “not applicable”. e 385 [NOx]0 represents the initial mixing ratios of NOx. For photooxidation experiments of gasoline and toluene, HONO 386 generated from the reaction of 0.1 M NaNO2 and 10% w/w H2SO4 was injected to the chamber as a source of OH radicals. 387 The concentration of HONO was estimated using the decrease of NO2 signal in the presence of a base denuder (coated 388 with 1% Na2CO3+1% glucose). f 389 The SOA yield of gasoline was calculated using the maximum SOA concentration divided by the total consumption of 390 aromatic HCs, including toluene, (o, m, p)-xylene, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene. g 391 The RH and temperature conditions for each experiment were recorded from sunrise to the collection of the final sample. h 392 The SOA samples were applied to DTT assay, organic hydroperoxides analysis (OHP), PAN analysis, GC-MS analysis, 393 or FTIR analysis. 394

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Figure 1. (a) The DTTm of wood smoke particles (high-temp combustion and smoldering combustion). The lines in each figure represent the time-based linear regression of initial DTT consumptions. (b) The aging effect on DTTm, OHP and PAN. The grey-dash line represents the completion of chemical injection into the chamber. (c) DTTm from OHP and PAN.

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Figure 2. (a) The DTTm of SOA derived from gasoline, toluene, isoprene and α-pinene under different NOx conditions (HNOX: high NOx, LNOX: low NOx). The lines in each figure represent the timebased linear regression of initial DTT consumptions. (b) The aging effect on DTTm, OHP and PAN. The grey-dash line represents the completion of chemical injection into the chamber. (c) DTTm from OHP and PAN.

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Figure 3. The time profiles of quinones (including PQN and AQN) concentrations in wood smoke particles measured using GC-MS.

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Figure 4. (a) The concentrations of functional groups in LNOX-toluene (09/19/17) SOA at 15:00 EST were estimated using FTIR spectral data except for OHP (NPBA assay). (b) The comparison of DTTm and chemical compositions of LNOX-toluene SOA. The quantities of PAN and OHP in the SOA at 15:00 were estimated using the interpolation between the corresponding values at 13:30 EST and 15:50 EST. The concentration of electron-deficient alkenes was estimated by decoupling the FTIR spectrum.

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